Photography: the art or process of producing images by the action of radiant energy and especially light on a sensitive surface.
Light is a huge factor of capturing great photos. However, it can also make photos turn out less than expected by bothering the client’s eyes. Many people have very sensitive eyes that are overwhelmed by consistent flashes, glare, and bright sunlight. This can result in photos where folks are squinting, or worse, blinking! The last thing anybody wants is for the client to have their eyes closed in all of the photos. Not to mention, the best photos happen when the client is stress-free and comfortable. Bright light sensitivity isn't something to be ashamed of, just something to be aware of and to plan for.
One thing that works very well, I have found, is for the client to get into an established pose, look into the camera lens, and then close one's eyes. Tell your photographer you are doing this, and then have them "count you in" by calling out "3, 2, 1" and then you open your eyes. This simple adjustment will help you to be prepared for the flash, and will minimize the amount of light reflected.
If the session is taking place outside, try to find a few shady areas. Doorways are great for this, as they allow sunlight in the photo, but a bit of shade on the client. If you are in an area where there is no shade, you can create shade with an umbrella or other prop. Also, looking down or at an angle is a comfortable pose to avoid direct sunlight in the eyes.
All in all, the best way to avoid bright flashes and sunlight is to be prepared. Think about all the factors, such as location, and create a plan with your photographer!